新疆北部哈纳斯盆地火山岩地球化学特征、锆石U-Pb年龄及其构造意义

Geochemistry and zircon U-Pb age of volcanic rocks from the Hanasi basin in the northern Xinjiang and their tectonic significance

哈纳斯火山-沉积盆地位于新疆阿尔泰造山带北部,盆地内的火山岩是一套钙碱性的中酸性岩石,为安山岩-英安岩-流纹岩组合。微量元素分析表明,哈纳斯火山岩大体上富集Rb、Sr、Th和La等大离子亲石元素,而亏损Zr、Y、Ti、Nb和Ta等高场强元素,并呈现相似的轻稀土元素中等富集的分布模式,表现出典型弧火山岩的特征。FeO、MgO、CaO、Al2O3、TiO2等主元素和Sr、Ni、Cr、Co等微量元素的含量与SiO2含量呈明显的线性相关关系,表明该区火山岩可能是同源岩浆分异的产物,岩石形成于活动大陆边缘的构造环境,并由遭受消减板块流体交代的富集地幔楔部分熔融而形成。英安岩单颗粒锆石U-Pb年龄为(474.5±4.7)Ma,与中—晚奥陶世地层年代相吻合。在这一时期新疆北部存在古亚洲洋,并向北部西伯利亚板块俯冲,形成了以哈纳斯火山-沉积盆地为代表的陆缘火山弧,这类陆缘火山弧是早古生代的中亚造山带典型的侧向大陆增生弧。

The Hanasi volcanic basin is located in the northern part of the Altai orogenic belt, Xinjiang, China. There is a calc-alkaline volcanic suite consisting of andesite-dacite-rhyolite in the basin. These volcanic rocks are generally enriched in LILE (Rb, Sr, Th and La) and depleted in HFSEs (Zr, Y, Ti, Nb and Ta). REE distribution patterns are characterized by a moderate enrichment of LREE. These trace element and REE characteristics are typical of arc volcanic rocks. The content of major elements, such as Fe, Mg, Ca and Al, and trace elements, such as Ti, Sr, Ni, Cr and Co, is linearly correlated to that of SiO2, which suggests that the different volcanic rocks in the basin should be the product of fractional crystallization of congenetic magmas. The parental magma may be generated by the partial melting of an enriched mantle wedge formed by the metasomatism of slab-derived fluids in an active continental margin tectonic setting. U-Pb single-grain zircon age of the dacite is (474.5±4.7) Ma, which agrees with stratigraphic chronology. During this period, the Paleo-Asia Ocean in North Xinjiang subducted northward under the Siberia plate, which resulted in a series of continental volcanic arcs. The Hanasi volcano-sedimentary basin is representative of the typical early Paleozoic lateral continental accretion arc in the Central Asian orogenic belt.